At the start of "The Great Journey" push Rtas 'Vadumee off the cliff and kill the Spectre driver behind you, but be careful not to destroy the Spectre. Then proceed through the level as you normally would until you get to the part where you help Johnson's Scarab clear the way to the control room, then carefully push a Ghost the cliff facing the control room. Now, drive off the cliff in the Spectre, to survive the fall. Enter the Ghost and drive to the back of the control room. There should be a platform with a beam next to it. Not to be confused with the larger beam on the side of the structure. Carefully drive the ghost up on the beam pushing against the wall and continue till you get to the front of the structure and wait for the Scarab to blast the door below you then drive down the slope and carefully fall to the control room door. Squeeze the vehicle through the destroyed hallway until you get to the main control room and let the cutscene play. Now go back to the hallway, where your Ghost should be waiting.

Getting a Banshee into the control room is much simpler than the two ground vehicles. To do it, just play through the level until Johnson uses his Scarab to blow open the control room door. From there, squeeze your Banshee through the damaged hallway and proceed through to the main control room, where a cutscene will play. When it finishes, your Banshee should be in the hall behind you.